1. Field of the Invention
The present invention pertains to wireless or mobile telecommunications system which utilize multiple access type technologies, and particularly to a channel selection scheme for a multiple technology access type system, e.g., CDMA/TDMA system.
2. Related Art and Other Considerations
In wireless or mobile telecommunications systems, user equipment in the form of a mobile station (e.g., mobile telephone) is typically in radio communication over an air interface with a base station. There are numerous techniques for providing access to the mobile telecommunications system for multiple users, e.g., multiple mobile stations. One general technique (known as frequency division multiple access [FDMA]) employs plural carrier radio frequencies, and assigns each user a different radio frequency. But since each simultaneous radio frequency transmission requires one radio, other techniques are often utilized to decrease cost and increase the flexibility of offered services (either alone or in conjunction with FDMA). One such other technique is time division multiple access [TDMA], wherein each carrier frequency is conceptualized as carrying frames of information, each frame then being divided into time slots or channels, with differing users having differing time slots. Typically (for duplex) there needs to be an xe2x80x9cuplinkxe2x80x9d time slot or channel for sending traffic from the mobile to the base station and a xe2x80x9cdownlinkxe2x80x9d time slot for sending traffic from the base station to the mobile.
One example technology which combines FDMA and TDMA (multi-carrier TDMA [MC/TDMA]) operates in Europe in the spectrum of 1880-1900 MHz under the name DECT. Aspects of DECT are described in ETR 310: August 1996, Annex E: DECT Instant Dynamic Channel Selection (DCS) Procedures, pp. 69-74. DECT installations are primarily intra-company mobile telecommunications systems which typically are quite limited in geographical range. In DECT, mobile stations are always locked to a closest (strongest) base station. After having locked itself to a strongest of the base stations, the mobile station makes a list of least interfered channels which it regularly updates. When the user of the mobile station desires to make a call, the mobile station selects the best channel and sends an access request (RACH) message to the strongest base station. This request is sent in synchronism with the base station receiver RF carrier scanning order. If a response is received on the relevant duplex response slot, the duplex channel is established. In DECT, handover is controlled by the mobile station. An automatic or seamless (xe2x80x9cmake before breakxe2x80x9d) handover is made as soon as another base station becomes stronger. DECT provides paging and system information on every downlink channel.
Mobile telecommunications service can be provided using yet another multiple access technique known as code division multiple access (CDMA). In a code division multiple access (CDMA) mobile telecommunications system, the information transmitted between a base station and a particular mobile station is modulated by a mathematical code (such as spreading code) to distinguish it from information for other mobile stations which are simultaneously utilizing the same radio frequency. Thus, in CDMA, the individual radio links are discriminated on the basis of codes. In addition, in CDMA mobile communications, typically the same baseband signal with suitable spreading is sent from several base stations with overlapping coverage. The mobile terminal can thus receive and use signals from several base stations simultaneously. Moreover, since the radio environment changes rapidly, a mobile station likely has radio channels to several base stations at the same moment, e.g., so that the mobile station can select the best channel and, if necessary, use signals directed to the mobile from various base stations in order to keep radio interference low and capacity high. This utilization of radio channels to/from multiple base stations by a mobile station, such as occurs in a CDMA scheme for example, is termed xe2x80x9csoft handoverxe2x80x9d or xe2x80x9cmacro diversity.xe2x80x9d In some geographical areas, FDMA/CDMA or MC/CDMA is utilized in the frequency spectrum beginning at 1920 MHz. Various aspects of CDMA are set forth in Garg, Vijay K. et al., Applications of CDMA in Wireless/Personal Communications, Prentice Hall (1997).
Traditional Direct Sequence CDMA (DS-CDMA) mobile radio systems require careful coordination of all systems using a common spectrum. DS-CDMA systems require rapid and accurate up-link power control of all mobile stations within a cell. Accordingly, operation of geographically overlapping DS-CDMA system is problematic. Thus, typically DS-CDMA does not have more than one system controlling the transmit power of mobile stations within a given cell and same spectrum.
Subject to the foregoing concerns, if overlapping DS-CDMA systems were configured and discriminated on the basis of spreading codes, the systems likely would not have sufficient capacity. In this regard, while a first DS-CDMA system might be assigned a first subset of the spreading codes and a second DS-CDMA system be assigned a second subset of the spreading codes, it should be kept in mind that generally the number of usable codes is relatively small (on the order of 50-100, for example). Moreover, this constraint is particularly onerous if more than two DS-CDMA systems were to overlap, or if expansion of further DS-CDMA systems is desired in the same geographical region. Traditional DS-CDMA mobile radio systems need careful coordination of all systems using a common spectrum, since different systems (without handover between them) can not operate in the same geographical area. The reason is that the key functionality of DS-CDMA systems is based upon equal down-link power for all connections and quick and accurate up-link power control of all handsets within a cell. Therefore, it is not allowed (on the same frequency) within a cell of a system to have one or more handsets, whose transmit power is not under the control of that system. This is in popular terms called the near/far problem. Therefore, in order to accommodate a large number of mobile stations, e.g., in a large geographical area, system discrimination merely on the basis of spreading codes is inadequate.
Opportunities exist for development of other segments of the frequency spectrum, such as 1900-1920 MHz in Europe, for example. What is particularly desired is to have plural uncoordinated DECT-like systems operating in an overlapping fashion, e.g., in larger than traditional DECT-sized geographical areas. It is contemplated that future technologies might employ a marriage of all three techniques: TDMA, FDMA, and CDMA. One main advantage of DS-CDMA systems, compared to MC/TDMA systems, is that it is possible to make a trade off between range and traffic capacity. Yet is not clear how these three techniques can interwork effectively, particularly since the sharing of spectrum resources between uncoordinated systems installed in a same geographical area is possible only in the frequency and time domains (but not the code domain due, e.g., to the above-mentioned near/far problem).
Traditional direct sequence DS-CDMA mobile radio systems need careful coordination of all systems using a common spectrum, since different systems (without handover between them) can not operate in the same geographical area. The reason is that the key functionality of DS-CDMA systems is based upon equal down-link power for all connections and quick and accurate up-link power control of all handsets within a cell. Therefore, it is not allowed (on the same spectrum) within a cell of a system to have one or more handsets, whose transmit power is not under the control of that system.
Application of various kinds of Instant Dynamic Channel Selection procedures (where the channel selection is made on per-call bases) are known, e.g. for MC/TDMA systems like DECT, which provides for uncoordinated system installations on a common spectrum. IDCS for DS-CDMA does however require specific solutions to address the new problems due to addition of CDMA. One main advantage of DS-CDMA systems, compared to MC/TDMA systems, is that it is possible to make a trade off between range and traffic capacity.
What is needed, therefore, and an object of the present invention, is a channel selection scheme for uncoordinated systems in a same geographical area which utilize multiple technology access types.
One of the many advantages of the present invention is that it also applies to CDMA systems with processing gain that is so low that the typical reuse factor 1 cannot be used. For this case the cumbersome channel planning needed for the different cells is made automatic.
A first telecommunications system has at least partially geographically overlapping coverage with a second telecommunications system, with radio transmission access in the first telecommunications system utilizing code division multiple access (CDMA) with a time division multiple access (TDMA) component. A base station of the first telecommunications system communicates over an air interface with a subscriber station and broadcasts a common broadcast physical channel on at least an active carrier/timeslot combination. The common broadcast physical channel includes broadcast system identity information. The subscriber station uses the broadcast system identity information to distinguish between transmissions of the first telecommunications system and the second telecommunications system. Moreover, the subscriber station uses the broadcast system identity information for framing transmissions in the first telecommunications system.
The base station of the first system broadcasts at least two common broadcast physical channels active on different timeslots. Preferably, a fixed direct sequence CDMA code is employed for the common broadcast physical channel.
In accordance with an instant dynamic channel selection procedure of the present invention, the subscriber station selects a carrier/timeslot for sending an access request to the first system base station. The carrier/timeslot selected by the first system subscriber station for sending an access request to the first system base station is also used for a traffic channel. Preferably, the first system subscriber station selects a least interfered carrier/timeslot as the carrier/timeslot for sending an access request to the first system base station. In response to the access request, the first system base station sends an answer on a next down-link carrier/timeslot included in a duplex pair with the carrier/time slot which the first system subscriber station selected for sending the access request to the first system base station.
At an given moment a connection between the first system base station and the subscriber station is borne on a physical channel of the first telecommunications system, the physical channel being defined by a code, timeslot, and frequency.